1. Field of the Invention
The invention relates to a plasma accelerator arrangement having a plasma chamber around a longitudinal axis, having an electrode arrangement for producing an electric acceleration field for positively charged ions over an acceleration section parallel to the longitudinal axis, and having means for introducing a focused electron beam into the plasma chamber and guiding it by means of a magnet system.
2. Prior Art
U.S. Pat. No. 5,359,258 A shows a plasma accelerator arrangement in the form of a Hall thruster, as it is known, having an annular acceleration chamber and a substantially radial magnetic field through the plasma chamber. The anode and anode-stage part of the plasma chamber are magnetically shielding. A gas is introduced into the plasma chamber, which is open on one side in the longitudinal direction, said gas being ionized by electrons and accelerated away from the anode and expelled said electrons coming from a cathode located outside the plasma chamber and being accelerated toward an anode located at the foot of the plasma chamber. The radial magnetic field forces the electrons on closed circular paths around the longitudinal axis of the arrangement and therefore increases their residence time and collision probability in the plasma chamber.
In an ion source which is disclosed by JP 55-102 162 A, in which an annular anode encloses a permanent magnet and, in turn, is surrounded by a circularly cylindrical cathode, a hollow ion beam is expelled from an annular opening.
An arrangement for producing ions with a high kinetic energy, of the order of magnitude of 10 GeV, for physical experiments is disclosed by U.S. Pat. 3,626,305. Here, a ring current of low-energy electrons at 10 MeV, for example, is produced outside an annular vacuum chamber and injected into the compression chamber. From a gas introduced briefly in pulse form, a number of positive ions which is low as compared with the number of ring electrons is produced by means of ionization and is caught in the potential head produced by the electron ring. By means of a strong, briefly pulsed magnetic field, the electrons circulating in the ring are accelerated highly to a ring current of, for example, 50 k-amp. The high magnetic field parallel to the axis of the and associated with the ring current of high-energy electrons comes to interact with a magnetic field produced in the vacuum chamber by inner and outer coils, so that the ring current is highly accelerated in the axial direction. The ions caught in the potential cup of the compressed electron ring system are carried along axially with the ring current and, as a result, are accelerated to a high kinetic energy. In U.S. Pat. No. 3,613,370, a plasma accelerator is described in which an annular plasma chamber is penetrated by a substantially radially oriented magnetic field. Electrons from a central cathode are guided into the plasma chamber through lateral openings in the inner wall of the plasma chamber.
GB 2 295 485 A shows an arrangement for producing an accelerated plasma jet in which, in a cylindrical plasma chamber, electrons emitted by a central cathode are accelerated in the direction of a ring anode. A magnetic field is used to prolong the residence time of the electrons in the plasma chamber in order to improve the ionization efficiently.
U.S. Pat. No. 4,434,130 describes the guidance of two oppositely directed accelerated ion beams from a fusion reactor by means of the space-charge effect of hollow cylindrically guided electrons. The guidance of the electrons moved on spiral paths is carried out with force equilibrium between radially oriented electrostatic fields and centrifugal forces. The ion beams supplied in the axial direction from both sides collide with high energy in the fusion region, whereas the electron beam supplied on one side with conical compression is widened again at the other end and is guided away.
DE 198 28 704 A1 discloses a plasma accelerator arrangement having a plasma chamber around a longitudinal axis, having an electrode arrangement and a magnet system as well as means for introducing an electron beam into the plasma chamber.
In this known arrangement, a circularly cylindrical plasma chamber is provided, in which a strongly focused electron beam generated by a beam generation device is introduced along the longitudinal axis of the cylinder. The electron beam is guided along the cylinder axis by a magnet system which, in particular, can be characterized by alternate polarization of the successive sections. The electrons of the electron beam, introduced into the plasma chamber at high velocity, pass through an electrical potential difference along the longitudinal axis of the plasma chamber, the difference having a decelerating action on the electrons of the electron beam. An ionizable gas, in particular a noble gas, is fed through the plasma chamber and is ionized by the electrons of the electron beam introduced and by secondary electrons. The positive ions produced in the process are accelerated along the longitudinal axis of the plasma chamber by the potential difference and move in the same direction as the introduced electron beam. The ions are likewise guided along the longitudinal axis, focused by the magnet arrangement and by space charge effects and, together with part of the electrons of the electron beam, emerge at the end of the plasma chamber in the form of a neutral plasma beam.
The present invention is based on the object of specifying a plasma accelerator of this type with a high efficiency.
According to the present invention, the electron beam is not introduced into a circularly cylindrical plasma chamber as a sharply focused beam; instead, for example via an annular cathode surface, a hollow cylindrical beam is produced, which is introduced into a toroidal plasma chamber. The plasma chamber is bounded radially by an outer chamber wall and an inner chamber wall and the hollow beam, with a wall thickness that is lower than the radius of the hollow cylinder, is fed in between these walls and guided by a magnet system. The entire arrangement is preferably at least approximately rotationally symmetrical or at least symmetrical in rotation about a longitudinal axis of the arrangement. The magnet system preferably likewise has a dual toroidal structure with a first magnet arrangement located radially on the outside with respect to the plasma chamber and a second magnet arrangement located on the inside.
As is already the case in the known arrangement, the arrangement according to the invention preferably also contains at least one intermediate electrode in the course of the plasma chamber, in the longitudinal direction, the intermediate electrode being at an intermediate potential of the potential difference along the longitudinal direction of the plasma chamber. The subdivision into a plurality of intermediate potentials permits a considerable improvement in the efficiency, by electrons of low kinetic energy being intercepted at an intermediate electrode with a potential difference that is lower than the current potential of an electron. The efficiency increases monotonically with the number of intermediate potential stages.
In a first embodiment, the magnet system can be designed in one stage with a pole change in each case for the outer and the inner magnet system, by means of opposed magnetic poles spaced apart in the longitudinal direction. At least one of the two magnetic poles in each case is located in the region of the plasma chamber in the longitudinal direction. Both poles of the single-stage magnet system, spaced apart in the longitudinal direction, preferably lie within the longitudinal extent of the plasma chamber. Particularly advantageous is an arrangement in which the magnet system is of multi-stage design having a plurality of successive subsystems in the longitudinal direction, each of which has an outer and an inner magnet arrangement and in which the successive subsystems in the longitudinal direction are alternately aligned in opposite directions.
Particularly beneficial is a plasma accelerator arrangement according to the invention in which, in the longitudinal course of the plasma chamber in the region of the side walls of the plasma chamber, there is still at least one intermediate electrode arrangement which is at an intermediate potential of the potential difference for accelerating the positive ions or retarding the introduced electron beam. On such an intermediate electrode, electrons which have only a low kinetic energy can be intercepted. The potential difference between cathode and anode can as a result be subdivided into two or more acceleration potentials. Losses due to electrons accelerated against the introduced electron beam can be reduced significantly as a result. In particular, the electrical efficiency increases monotonically with the number of potential stages. The electrodes in the longitudinal direction are advantageously in each case placed between the ends of poles of a magnet system or magnet subsystem. This results in a particularly beneficial course of electric and magnetic fields.